Material that has accumulated in a wellbore before or during completion is often called wellbore fill; it may be sand, proppant, cement chunks, or other materials. Such materials hinder or prevent operations. Sand accumulated in production tubing in a wellbore at the start of or during production can greatly hinder production. Coiled tubing has been widely used as a means to clean out the wellbore or production tubing in these situations. Coil tubing cleanouts of wellbores are commonly performed to remove “wellbore fill.” Such fill may comprise sand, proppants, cement chunks and debris from previous wellbore operations. Coiled tubing equipment is commonly used as the cleanout equipment. Such cleanout operations utilize fluids pumped down the wellbore through coiled tubing to pick up solid particles and then transport the particles back to the surface. To be effective at entraining and carrying solid particles, fluids used in cleanout operations are often viscosified. Most typically, however, these viscosified fluids are discarded after a single trip through the well because removal of the entrained solids from the fluid either requires a long “settling time” or the addition of a chemical “breaker” to reduce the fluid viscosity.
Requirements for fluids used for wellbore fill removal, more particularly “coiled tubing cleanout”, include low friction pressure and good particle carrying capability. Some crosslinked polymer gels are not preferred viscosifiers for cleanout fluids because they do not have preferred viscoelastic properties for fluidizing the particles so that they can be transported. Cleanout systems that can be recycled have not been reported, and would be highly desirable for logistical and operational reasons, especially in offshore locations. Such recyclable cleanout systems also give cost benefits and are environmentally friendly.